After some investigation it appeared that when I was testing the filter and running the cream pbs qsub submit by hand I was always setting the number of nodes, even if I only required 1 i.e.
as a pool account ....
/opt/glite/bin/pbs_submit.sh -c /bin/hostname -q q1d -n 1
This meant that there was always a #PBS -l nodes=
As it turns out on deeper investigation into the CREAM pbs_submission script. That when 1 node is required it uses the pbs default behaviour and does not specify a number of nodes. Only when there is more than one does it specify this i.e. MPI. This is a change from the lcg_CE job manager which always specifies a number of nodes be it 1 or more. Something to remember.
To get round this I have added an additional line to the cream pbs submit script to always default to 1 node if not MPI. Not the best but it's a short lived tweak until we get rid of our SL4 support. This should be very soon.
[ ! -z "$bls_opt_mpinodes" ] || echo "#PBS -l nodes=1" >> $bls_tmp_file